1. Field of the Invention
This invention relates to a charging device for charging a member to be charged, such as a photosensitive member, a dielectric member or the like, an image forming apparatus which uses this device, and a process cartridge which is detachable to this apparatus.
2. Description of the Related Art
Heretofore, in an image forming apparatus, such as an electrophotographic apparatus (a copier, a laser-beam printer or the like), an electrostatic recording apparatus or the like, noncontact-type charging means, in which a corona discharging unit including a wire and a shield is used, and the surface of a member to be charged, such as an image bearing member (for example, a photosensitive member or a dielectric member) or the like, is exposed to corona generated by the unit, have been widely used as means for performing charging processing (including charge-removing processing) of the member to be charged.
Recently, contact-type charging means, which perform contact charging, have been more and more adopted. In contact charging, a voltage is applied, for example, to a roller-type or blade-type charging member (a contact charging member comprising a conductive member), and the surface of a member to be charged is charged by making the charging member in contact with or in the proximity of the member to be charged.
The charging member need not always contact the surface of the member to be charged, and may not contact (or be in the proximity of) the surface of the member to be charged, provided that a chargeable region, which is determined by the gap voltage and the correction Paschen curve, can be secured between the charging member and the member to be charged.
Contact-charging or proximity-charging units have, for example, the following advantages compared with noncontact-charging corona charging units. That is, the value of an applied voltage necessary for obtaining a desired potential on the surface of a member to be charged can be reduced. The amount of ozone generated in a charging process is very small, and therefore the use of an ozone-removing filter is unnecessary. Hence, the configuration of an exhaust system of the apparatus can be simplified. A maintenance-free apparatus can be provided. The configuration of the apparatus can be simplified.
As previously proposed by the assignee of the present application (for example, Japanese Patent Application Laid-open (Kokai) No. 63-149669 (1988)) with respect to contact charging, a method of performing charging by applying an oscillating voltage (a voltage whose value periodically changes with time), more particularly, an oscillating voltage whose peak-to-peak voltage is at least twice the charging-start voltage of a member to be charged when a DC voltage is applied (hereinafter termed an AC application method) can perform uniform charging (including charge-removing) processing, and therefore is effective.
FIG. 9 illustrates the schematic configuration of an image forming apparatus which adopts a contact charging unit of the above-described AC application method as charging means for an image bearing member. The apparatus comprises a laser-beam printer which utilizes an electrophotographic process.
A drum-type electrophotographic photosensitive member (hereinafter termed a photosensitive drum) 1, serving as a member to be charged, is rotatably driven at a predetermined peripheral speed (process speed) in a clockwise direction, as indicated by arrow A.
Charging roller (conductive roller) 20, serving as a charging member, comprises a metal core bar 21, and a conductive roller member 22, made of conductive rubber or the like, formed at the outer circumference of metal core bar 21. Charging roller 20 is in pressure contact with the surface of photosensitive drum 1 with a predetermined pressure given by pressing springs 23 provided at both end portions of metal core bar 21. In the present case, charging roller 20 is rotatably driven in accordance with the rotation of photosensitive drum 1.
Reference numeral 4 represents a power supply for applying a voltage to charging roller 20. Power supply 4 applies a superposed voltage (V.sub.ac +V.sub.dc), comprising a AC-component voltage V.sub.ac, whose peak-to-peak voltage equals at least twice the charging start voltage for photosensitive drum 1, and a DC-component voltage Vdc, to charging roller 20 via contact leaf spring 3 contacting metal core bar 21 of charging roller 20, whereby the outer circumferential surface of the rotatably-driven photosensitive drum 1 is subjected to uniform contact charging by the AC application method.
On the other hand, a time-serial electrical digital pixel (picture element) signal of target image (printing) information is input from a host apparatus (not shown), such as a computer, a word processor, an image reading apparatus or the like, to a laser scanner (not shown). The laser scanner controlled by a controller outputs laser light 5 subjected to image modulation with a constant printing density D.sub.dpi in accordance with the input pixel signal. By performing line scanning (main-scanning exposure in the direction of the generatrix of the drum) of the output laser light 5 for the charged surface of the rotating photosensitive drum 1, the target image information is written to form an electrostatic latent image of the image information on the surface of the rotating photosensitive drum 1.
The latent image is visualized as a toner image by performing reversal development using developing sleeve 6 of a developing unit. The toner image is sequentially transferred onto transfer material 7 fed from a sheet-feeding unit (not shown) to a pressure-contact nip portion (transfer portion) between photosensitive drum 1 and transfer roller 8 with a predetermined timing.
Transfer material 7 onto which the toner image has been transferred is separated from the surface of photosensitive drum 1 and conveyed to fixing means (not shown), where the toner image is fixed. Transfer material 7 on which the toner image has been fixed is output as an image-formed material. The surface of the rotating photosensitive drum 1 after separating transfer material 7 is cleaned by removing any remaining deposit, such as remaining toner after transfer, or the like, using cleaning blade 9 of a cleaner, and is repeatedly used for image formation.
The above-described image forming apparatus which utilizes a charging unit of the AC application method as charging means for an image bearing member, such as a photosensitive drum or the like, has the following problem.
That is, as shown in FIG. 12, when an image having lateral-line pattern 14a indicated by solid lines (reference numeral 14 represents recording paper) is output, if the interval of lateral-line pattern 14a is close to the interval of so-called "cycle pattern" 14b indicated by broken lines in the surface potential of the photosensitive drum which is determined by the frequency of the AC component of the voltage applied to a member to be charged from the power supply, interference fringes (a moire pattern) 14c appear on the image surface.
The frequency f of the AC component of the power supply has variations of plus or minus 10% from a determined value due to insufficient accuracy in the components, or the like. Accordingly, the frequencies of some power supplies become close to the spatial frequency of lateral-line pattern 14a, causing generation of distinct interference fringes 14c.
In order to overcome the above-described problem, a method may be considered in which the frequency of the AC component of the power supply is increased in accordance with an increase in the process speed. However, a recent increase in the process speed in accordance with a tendency toward high-speed image forming apparatuses causes an increase in so-called "charging tone" generated with the frequency of the primary power supply in accordance with an increase in the frequency of the primary power supply.
The peak-to-peak interval of the cycle pattern increases and therefore becomes noticeable when the process speed is high or the frequency of the primary power supply is relatively small, since the pitch of charging and discharging in the surface potential of the photosensitive drum caused by the charging member increases.